In recent years, in the orthogonal frequency division multiplexing (hereinafter called “OFDM (Orthogonal Frequency Division Multiplexing)”) mode, which is one of the multicarrier modulation modes, an orthogonal frequency division multiple access (hereinafter called “OFDMA (Orthogonal Frequency Division Multiple Access)”) mode in which multiple accesses are implemented by assigning users to respective subcarriers, or respective subchannels that are obtained by grouping a plurality of subcarriers has been attracting attention. This OFDMA mode has been studied, for example, in IEEE802.16e, which is a standard of IEEE (Institute of Electrical and Electronics Engineers).
In the OFDM mode, information is transmitted by using N subcarriers having different frequencies. Note that in the multipath fading environments where transmission signals are received after passing through various paths, since the radio qualities of subcarriers are different from each other, a subcarrier(s) having a good radio quality is different from one mobile station to another. Therefore, it is necessary to allocate a subcarrier(s) that is determined to have as good radio quality as possible to each mobile station. Note that the better the radio quality of a subcarrier is, the higher transmission rate the communication can be performed at. Therefore, if each mobile station is allocated with a subcarrier having as good radio quality as possible, the communication can be performed with excellent efficiency as the entire system.
As a method in the related art, a method in which the dispersion value of the transmission rates of subcarriers is calculated for each mobile station and then subcarriers are allocated to a mobile station having the largest dispersion value until the requested transmission rate is satisfied has been proposed (e.g., see Patent document 1).
The subcarrier allocation method disclosed in Patent document 1 is explained hereinafter by using a specific example. In this example, the number of mobile stations is three, and the number of subcarriers is five. Further, in this example, the transmission rate is calculated based on the modulation mode and the encoding rate according to a table shown in FIG. 11.
Firstly, the transmission rates of respective subcarriers and their dispersion value are calculated according to the table shown in FIG. 11 for each mobile station. FIG. 12 is a table showing transmission rates for respective subcarriers, the dispersion value of the transmission rates, and a transmission rate requested from a mobile station for each of three mobile stations.
Firstly, among the three mobile stations, the allocation of a subcarrier(s) is started from the third mobile station having the largest dispassion value. Then, among the subcarriers of the third mobile station, the subcarriers 3 and 4 having the highest transmission rate are preferentially allocated. When the subcarriers 3 and 4 are allocated, the total transmission rate becomes 3+3=6 and the requested transmission rate, which is 6, is thereby satisfied. Therefore, the subcarrier allocation for the third mobile station is finished.
Further, the subcarrier allocation is performed for the first mobile station having the next highest dispersion value after the third mobile station. Then, since the subcarriers 3 and 4, among all the subcarriers of the first mobile station, were already allocated to the third mobile station, the subcarrier allocation is performed from a subcarrier having the highest transmission rate among the remaining subcarriers 1, 2 and 5. In this example, when the subcarriers 1, 2 and 5 are allocated to the first mobile station, the total transmission rate becomes 1+1+1.5=3.5 and thereby satisfying the requested transmission rate of the first mobile station, which is 3. Therefore, the subcarrier allocation for the first mobile station is finished. With this allocation, all the subcarriers 1 to 5 have been allocated, and therefore the allocation is finished. FIG. 13 is a table showing the allocation result.    [Patent Document 1]    Japanese Unexamined Patent Application Publication No. 2005-102225